LONDON—Roche Holding AG said Sunday it would pay $8.3 billion for a California biotech firm that has yet to turn a profit on a new drug to treat a deadly lung disease—the latest gamble by a pharmaceutical giant to buy its way into a lucrative corner of the industry.

The takeover would allow the Swiss company to expand its presence in the treatment of respiratory disorders, one of the world's biggest drug markets. Roche's offer of $74 a share represents a 38% premium over InterMune's closing share price on Friday of...